a) Field of the Invention
The present invention relates to an apparatus for growing a GaAs single crystal by pulling up from a melt thereof.
b) Description of the Prior Art
As well known, single crystals of GaAs which is a compound semiconductor are used as the substrates for the fabrication of various semiconductor devices such as light-emitting diodes, laser diodes or microwave diodes, and such crystals are used also as the material for fabricating semiconductor devices such as transistors and semiconductor integrated circuits which are operated at high speeds, in view of the fact that GaAs has an electron mobility of 8500 cm.sup.2 /V.sec which is more than 5 times greater than that of silicon. As a matter of course, it is desired that GaAs single crystals which are employed as the substrates for the fabrication of those devices as mentioned above have a superior crystal habit. Conventionally, GaAs single crystals have been grown by relying on, for example, horizontal Bridgman method and or the LEC (Liquid Encapsulated Czochralski) method in which the growth zone is sealed off with a B.sub.2 O.sub.3 capsule. The Bridgman method employs a quartz tube, which leads to such drawbacks that there is a difficulty of obtaining a single crystal of a large diameter, and that, since it has been usual to grow a crystal in the (111) orientation, there is the necessity to cut the crystal obliquely to the (111) orientation instead of normal thereto in order to obtain a substrate having a (100) face which frequently is used in various semiconductor devices, and this necessity results in the lack of uniformity of the impurity concentration in the substrate, and further that although a semi-insulating, i.e. highly resistive, substrate can be obtained usually by doping an impurity such as Cr or O or both, such an impurity requires to be doped at a high temperature of around 800 .degree. C. or above, and it cannot be doped with stability. The LEC method, on the other hand, enables one to obtain a crystal of a large diameter in the (100) face, but since it employs a B.sub.2 O.sub.3 layer as the sealing or encapsulating member, this brings about a large temperature gradient of about 100.degree..about.300.degree. C./cm between the GaAs melt and the seed crystal, so that defects such as lattice dislocation tend to be induced during the crystal growth. Besides these conventional methods, there has been practiced such method as Czochralski's pulling method. This latter method, however, has drawbacks similar to those mentioned above. There are further important problems about GaAs crystals, as will be discussed below.
Unlike Si crystals, a GaAs crystal which is a compound semiconductor is made by compounding two elements of Ga and As. In addition, the vapor pressures of the Ga element and of the As element have a great divergence over each other, and the vapor pressure of As is by far the greater than that of the vapor pressure of Ga. Thus, the GaAs crystal which is grown by the conventional methods exhibits a considerably great deviation from stoichiometric composition. In the usual growth process, As atoms tend to present a shortage within the GaAs crystal, leading to the development of defects such as As vacancies or lattice dislocation, or the defect due to combination of As vacancies and the atoms of an impurity. Recent researches have made it clear that a deviation of the crystal being grown from stoichiometric composition most greatly affects the characteristics or the life of the device fabricated by this crystal. The present inventor earlier proposed a method of solving this problem in the technical field of epitaxial growth of compound semicondcutor crystals by supplying the vapor of As during the growth process, with a very high degree of success. In the technical field of producing a single crystal by relying on the pulling method, however, there has not been considered a growth apparatus in which the problem of deviation from stoichiometry is taken into account.